The Role of Thermal Modeling in the Design of Electronic Equipment

By Prabhu Sathyamurthy, Fluent Incorporated


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•How Thermal Modeling Saves Money

Electronic equipment design has changed dramatically over the last 25 years. The process of breadboarding a design, making modifications until the electrical connections are correct, and then committing the design to a printed circuit board has given way to computer aided design, rapid prototyping, and five month lead times. Adding a heat sink to the rare, unreliable component has given way to power-hungry microprocessors and peripheral devices which require sophisticated thermal management.

Thermal modeling is a helpful technique for integrating thermal design into the overall equipment design cycle. Once considered the exclusive tool of thermal experts, thermal modeling software has become an easy-to-use tool with a proven ability to reduce cost by shortening the design cycle.

Today's microprocessors and their associated power supplies, hard drives, advanced video chips, and other peripheral devices draw large amounts of power and, consequently, dissipate a significant amount of heat. These same devices are also heat-sensitive, requiring that the generated heat be removed in order to maintain reliability. Putting a heat sink on the chip and a fan and some vents in the chassis may result in a system where some devices are hundreds of degrees over specification. Additional cooling solutions range from inefficient, expensive cryogenic cooling loops to inexpensive, highly effective heat pipes. Using a thermal modeling program allows the designer to explore any and all of these thermal solutions in many different physical configurations, without ever putting together two pieces of hardware.

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The software technology of choice for evaluating thermal behavior is computational fluid dynamics (CFD). CFD software uses mathematical representations of individual components as a basis on which to build a model that predicts airflow, mass and heat transfer, and the resulting temperature, at each part of the electronic assembly.

The first step in thermal modeling is identifying the critical components. Today's CFD software includes preprogrammed components such as printed circuit boards, fans, vents, and heat sinks. The user can define these components with the characteristics of the proposed component set. Figure 1 shows how easy it is to select components and their characteristics in one popular CFD program. Alternatively, the user can import the physical specifications for a component from a CAD program. The CFD program will also need the thermal specifications for the component. If the physical specifications of a component are not available in CAD format, today's CFD programs still allow manual input of both physical and thermal specifications of any component.

Some geometries are more difficult to model than others. To make modeling of these complex components simpler, the component can be divided into small cells, creating a grid, or "mesh". The software can then solve the necessary fluid equations for each cell, combining the results to emulate the single object. The process used to create the mesh used to take days, or even weeks. Today's CFD software provides automatic meshing functions that allow the user to build a mesh that conforms very closely to even the most complex geometries in a fraction of the time. The pop-up screen in Figure 1 shows how easy it is to use a default or user-defined mesh. Automated meshing technology uses a cocooning technique to eliminate unnecessary cells in regions where refinement is not necessary. This technique saves time and improves the accuracy of the model.

Computer resources are no longer the limiting factor in a CFD program's ability to produce rapid solutions. Large real system designs that would have required mainframe or super-computing power only a few years ago can now be solved on a desktop system. As a result, the solvers in today's CFD programs provide faster, more accurate, and more robust solutions than before. These solvers are able to properly model the relevant physics involved in electronics cooling problems, such as turbulence, radiation exchange and anisotropic conductivities. The CFD algorithms have improved to the point where there is little need for user-intervention.

These programs provide the designer with easy-to-use, easy-to-interpret graphical images representing the calculation results. Primary results, such as airflow velocity, pressure, and temperature are presented using color, contour lines, and vector fields, and give the designer immediate feedback about the effectiveness of a thermal solution. To help the designer understand why a design succeeds or fails from a thermal perspective, derived quantities such as flow rates and heat transfer coefficients are available graphically as well. Text-based information is still available for detailed analysis.


How Thermal Modeling Saves Money

When an engineer is designing a new or improved piece of electronic equipment, there are three demands to keep in balance: the electrical demands of the circuit, the thermal demands of the components, and the end-user demands on the product as a whole. Once the capability requirements of the major system components have been defined, the designer begins thermal modeling. The software models several different component layouts rapidly, giving the designer a good idea how these components affect one another. This basic information ensures that thermal requirements form part of the basis for component placement before the design is set in stone.

Where thermal modeling saves the most time and expense is in refinement of the pre-prototype design. Using representations of the equipment components, the software can test many different configurations in a few hours or days. Figure 2 shows a model of a preliminary design for a 64-bit computer. Although the system has two fans and a vent for the PCI slots at the top of the computer, the PCI boards are still too hot at 200°C. Within less than a half an hour of modeling time, the designer added a fan specifically for the PCI area and moved the vent below the fan, as shown in Figure 3. To add the fan, the designer selected "fan" from a screen similar to the one shown in Figure 1, and entered the specifications for the fan. This step reduced the temperature from 200° to 140°C. The designer added a baffle as shown in Figure 4 just as easily, but this time the results were less promising. The baffle actually increased the temperature in the PCI area by about 10°C.

All of this modeling took very little time, especially compared with the cost to build and test a prototype. The final thermal design for this took 335 watts of heat out of the system without the need for expensive repetitive prototyping. These results clearly demonstrate the effectiveness of thermal modeling to reduce design costs, reduce time to market, and to reduce the cost of the final product.

In another case, modeling was used to understand the thermal characteristics of a graphic card and choose the right heat sink for the design. The initial model took a few minutes to build and solve. Adding the heat sink took another half-hour total time, resulting in the image shown in Figure 5. Although other less critical components in the system get as hot as 50°C (as shown by the red color), the critical IC stays only 20°C higher than ambient with the heat sink in place. In this example, spending only a few minutes modeling the system improved system reliability by reducing component temperatures, and prevented an overly hot IC from delaying time-to-market.

In this last example, a designer needed to ensure that a high power device was sufficiently cooled by the system fan. The details of the power supply components themselves were not critical to the model, so are represented together as a volumetric resistance in the upper right hand corner of Figure 6. The model shows that sufficient air flow is moving past the heatsink in the upper left-hand corner to keep the ASICS on the PCB cool. This problem took less than an hour to model and solve. The insurance offered by a robust thermal design, tested through CFD modeling, is a reliable system without endless prototyping.



These examples show how a relatively complex electronic system can be modeled rapidly using today's CFD software. The products that are modeled benefit from higher reliability, lower cost, and shorter time-to-market. The designer shortens the time to solution and reduces the need for prototyping. The overall design efficiency afforded by modeling is one that cannot be overlooked by anyone designing electronic equipment for today's market.